RE: Puzzler: How Does Humidity Affect Counting Efficiency?

["Wes Van Pelt" <WesVanPelt@att.net>]

Dale and Radsafers,



Yes, it is an interesting fact that humid air is less dense than dry air.

(If the effect were larger we would have "wet air balloons" instead of hot

air balloons!)



But the density of air can have absolutely no effect on liquid scintillation

beta and HPGe gamma counting efficiencies.



I am sticking to my theory that the higher counting efficiencies are due to

changes in the 120 volt a.c. electric power in the lab.



Wes



Wesley R. Van Pelt, PhD, CIH, CHP

Wesley R. Van Pelt Associates, Inc.





-----Original Message-----

From: owner-radsafe@list.vanderbilt.edu

[mailto:owner-radsafe@list.vanderbilt.edu] On Behalf Of

daleboyce@charter.net

Sent: Thursday, May 27, 2004 10:26 AM

To: Wes Van Pelt; John_Sukosky@DOM.COM; radsafe@list.vanderbilt.edu

Subject: Re: Puzzler: How Does Humidity Affect Counting Efficiency?



Hi all,



Most don't realize it, but at a given temperature and pressure humid air is

less dense than dry air.  It is not a big factor, but it exists.  I first

heard about it in junior high from a science teacher who was an ex-military

pilot.  Planes have a noticeably longer take off roll on humid days since

they have to reach slightly higher speed to take off..



If the distance between the window of your Tennelec and the sample is

significant, that could be a factor for alpha's but not beta's.



Dale





> John and Radsafers,

>

> Well, here is my second guess at the solution to this puzzler.

>

> But first, review the facts:

>

> The counting efficiency went up over 2 or 3 months as humidity went up.

> The backgrounds stayed the same.

> The correlation holds true for three TOTALLY DIFFERENT types of

instruments:

> liquid scintillation,

> semiconductor detector (HPGe), and

> gas flow proportional.

> All are pulse counting instruments with a lower level discriminator or a

> counting window (region of interest).

> All use electricity to generate a high voltage (on the PM tube and gas

> chamber) or a voltage (across the semiconductor detector).

>

> The only connection among these three counters is the electric power

supply.

> My guess is that, over several months, the voltage or current supplied to

> the three instruments changed.

>

> I suggest you measure and record the voltage supplied to these instruments

> over a period of time. Perhaps the voltage drops as your air conditioning

> units come on line as the season gets warmer. You could also get an

> Uninterruptible Power Supply (UPS) or isolation transformer that regulates

> the output voltage to a constant value (e.g., 117 volts ac) even if the

> input voltage drops.

>

> Best wishes,

> Wes

>





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